This application claims priority from Korean Patent Application No. 2003-78116, filed on Nov. 5, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a digital optical communication system, and more particularly, to non-return to zero (NRZ) modulation method.
2. Description of the Related Art
In a digital optical communication system, a sending end outputs optical signal data synchronized by its own clock and a receiving end recovers the received data. In a digital communication system, data recovery is to read input data at every instant pointed by the clock signal and to judge whether the input data value is “0” or “1”. Even if a clock frequency of the receiving end is a little bit different from a clock frequency of the sending end, data cannot be normally recovered. Therefore, almost all receivers use a clock for data recovery by extracting the clock from the input data not to have own clock.
Nowadays, in 2.5 Gbps and 10 Gbps electrical time division multiplexing (ETDM) optical transmission system, optical transmission using an NRZ modulation method is usually used. The NRZ modulation method has a simple configuration and price competition in comparison to the other modulation methods. However, since NRZ data does not have any frequency component corresponding to the clock signal, there's no straightforward method of extracting clock signal from NRZ data.
A clock extracting method, which is generally used in a digital communication system using the NRZ modulation method, uses a phase locked loop (PLL) component. However, considering systems, whose transmission rate is more than 10 Gbps such as 40 Gbps, it is very difficult to manufacture an electrical component, such as the PLL and also the price of the component becomes very expensive even though the component can be manufactured.